Psalms 101:1
1I will sing of loving kindness and justice. To you, LORD, I will sing praises.
Wesley: I will praise thee, O Lord, for thy mercy and justice, which thou hast so eminently discovered in the government of the world and of thy people; and I will make it my care to imitate thee herein. The song is praise and imitation together—steadfast love and justice held up as the pattern of rule.
David freely vows before the throne is fully his. Mercy and justice are God's first; the king's music is a living resolve to copy them. Grace enables the vow; the singer still must sing and then walk what he sings.
Calvin: to sing of mercy and of judgment is equivalent to declaring in solemn terms that he would be a just and an upright king. Unto thee, O Jehovah, will I sing psalms: he acknowledges that it was by the favor of God that he was appointed to so distinguished an office. He comprehends all princely virtues under these two particulars, mercy and judgment.
The song is first thanksgiving for a calling God gave, then a vow shaped by that gift. Mercy and justice are not private taste; they are the King's own virtues under which David will rule.